Metallic materials have shaped big industries in the fields of building materials, bridges, railroads, vehicles, automobile parts, etc. However, metals have been replaced with plastics such as FRP for the reduction of weight. Under these circumstances, the demand for metals has diversified, in recent years, because of the wide use of electronics and the rise of leisure industry, and also from the viewpoint of environment and energy. Various new metal materials have been developed from the perspective of the reduction of weight and the achievement of high functionality. The usage development is also in progress by applying composite functionality such as corrosion resistance and durability as well as electrical conductivity and thermal conductivity, which plastics cannot replace.
For metals, there is a bonding technology as a specific processing technology for surface treatment. Bonding is one of the very important processing technologies as well as cutting and bending. As bonding, there are mechanical bonding, adhesion, and welding. Its purpose is to achieve composite functions such as surface improvement, surface gradient, and lamination of substrates such as bulk metals and ceramics. For example, chromium plating is a typical example of anticorrosion surface treatment. Recently, however, a replacement technology for the surface treatment is in demand from the viewpoint of the wastewater treatment. In addition, for the improvement of sliding, the adhesion to the substrate is an issue for a new material such as DLC film and others.
Various materials have been investigated as laminating materials such as surface treatment material and cladding material in order to add characteristics such as toughness, antibacterial property, and high class touch as well as corrosion prevention, rust prevention, and wear resistance.
It has been known that allowing the metal to be amorphous is advantageous in achieving these functions. However, the amorphous state of conventional amorphous metal (amorphous alloy) is unstable and it easily crystallizes.
In recent years, as a metallic material to solve the problem, metallic glass was discovered and brought to attention. The metallic glass (glass alloy) may be said to be a kind of amorphous alloy in the broad sense. However, the metallic glass has been distinguished from the conventional amorphous alloy in that it shows a distinct glass transition and a stable supercooled liquid state in a wide temperature range. Lately, there is a view that the metallic glass is aggregates of nanocrystals. Thus, it is considered that the fine structure of the amorphous state of metallic glass is different from the amorphous state of the conventional amorphous metal.
For the bonding between metal and substrate, welding or pressure welding methods are mainly used. The interface affinity of both constitutions exerts a strong influence on durability such as adhesion strength and peeling resistance. In addition, because respective materials have characteristic thermal expansion coefficients, the matching of both thermal expansion coefficients are very important.
The thermal expansion coefficient of metallic glass is lower than that of metal owing to its metallographic structure, and metallic glass has good ductility and excellent interface forming capability. Thus, metallic glass is utilized for the bonding of metals by making full use of these characteristics of metallic glass.
In the Patent Literature 1 mentioned-below, for example, a bonding method is disclosed in which metallic glass is heated to the supercooled liquid temperature range and then metals are bonded with pressure. By this method, however, it is difficult to maintain face-to-face contact between the metal surfaces because of the crystallization and deformation of the metallic glass.
In the Patent Literature 2 mentioned-below, a bonding method is disclosed in which the steps of heating, pressurization, and cooling of metallic glass are stipulated in detail in order to solve the problems of the Patent Literature 1. However, this method is also a bonding method between metal bulks, and it cannot be satisfactorily used for various applications.
In addition, the prevention of crystallization is a big issue especially when an amorphous layer is formed on the substrate.
In the case of a conventional amorphous alloy, if the cooling rate of the melt is slow, a crystalline phase is formed and it is difficult to obtain a homogeneous amorphous layer. The formation of a crystalline phase is not desirable because there is a negative effect to corrosion resistance, etc. In the case of metallic glass also, a high-grade coating has not been achieved as with normal crystalline alloy and amorphous alloy.
Patent literature 1: Japanese Unexamined Patent Publication H 5-131279
Patent literature 2: Japanese Unexamined Patent Publication H 11-33746